Changeling is a harrowing story of one woman's nightmare when she tries to find her missing son. For screenwriters it shows the benefits, but also the difficulty, of combining the thriller with the social drama.

Drama is one of the great storytelling forms, but it has become rare in a Hollywood that requires all its movies to have blockbuster potential. Most writers of classic drama have moved to television where the level of writing has never been higher. But to get a drama made in film, you have to combine it with a more sensational genre that can pull in the big audience. Enter the thriller.

At first glance, this marriage of forms looks like a good idea. Social drama lets you explore human nature and social conflict in depth, while the thriller gives the story excitement, jeopardy and narrative drive. But these same qualities dictate virtually opposite story movements. Drama wants to slow down and dig deep, find the underlying causes and explore the subtleties of human character. Thriller wants to charge ahead, to find out who is attacking the hero. No subtlety here; it's yes or no, he did it or he didn't.

This is the crux of the problem that Changeling writer, J. Michael Straczynski, had to solve to make this film work. The story begins with the social drama. The hero, Christine Collins, returns home from work to find her son missing. Months later the police bring her a boy they claim is her son, even though she insists he isn't. Her desire is simple: she wants her son back. But that creates a big structural problem for the writer. She can't act on her desire. All she can do is repeat it to the corrupt cops. And while this generates anger in the audience at the arrogant injustice of the police who treat her as an incompetent child, it doesn't drive the story forward.

This early part of the film also highlights another flaw common to the social drama. The hero has no moral need. All immorality is located in the opposition, the corrupt cops. This creates a good vs. evil contrast that is the kiss of death for good social drama, and exacerbates the hero's position as a victim already established by her reactive desire line.

With the social drama line slowing to a halt, Straczynski introduces the investigation line and the story takes off with a burst of energy that's palpable. A cop in the same precinct as the corrupt captain follows up on a kid's claim that he helped a serial killer murder boys. The resulting investigation is not complicated, but it doesn't have to be. It gives the story a goal with clear action beats. The writer then cross-cuts this line with a thriller line that comes from the corrupt captain sending Christine to an insane asylum for claiming the police gave her the wrong boy.

Does this story have a double desire line, and therefore two spines? You bet. And it does feel like two movies that have been cobbled together. But we also see here a writer making a unique story work on its own terms. Most stories have one or more structural roadblocks built in; they are part of the animal. If, as a professional, you have to make a story work, you pull from your bag of techniques and get the job done. Straczynski knows that neither desire line will support the story on its own. But by cross-cutting them, he creates a track with enough narrative drive to take us to the end of the social drama.

But not without severe costs. The story's structural flaws prevent this from being a great social drama. Yes, the corrupt captain and chief of police are brought low and the mother is publicly redeemed. But subtlety and an exploration of deeper causes are nowhere to be found.

So what can we learn from Changeling? I've already mentioned the importance of making the story work, even if you have to break a few of the rules of good drama. The lesson of never letting perfection get in the way of success is always good for writers to remember. Changeling also shows us the power, and the difficulty, of combining drama with thriller. The key structural element is desire. If possible, try to turn the two desire lines - of drama and thriller - into one. Each line should help solve the other: investigating the crime should lead to deeper layers of the social conflict while the argument about the social issue should lead the hero to new clues about the crime.

Finally, Changeling shows us one of the keys to dramatizing a real-life story: finding the right frame. A true story must hit the same seven major structure steps as a fictional story. But a true story restricts you in how you find those steps, since you can't just create them from thin air. Instead you have to focus on the frame, where to begin and end the story, and that means you have to start by identifying your battle scene. In Changeling, everything comes to a dramatic head at the trial, actually a cross-cut between the trial of the killer and the trial of the LA police. This battle brings a convergence of the two lines that this real-life drama-thriller desperately needs.

If you are interested in how to tell any story with maximum dramatic power, regardless of genre, look at our 22 Step Great Screenwriting Class. For tips on advanced drama and structuring true stories, check out the Advanced Screenwriting Class.

He has created an excellent software programme called Blockbuster, which you can purchase through Raindance.

He is a writer/director whose feature, ALL-AMERICAN BOY, won Best Dramatic Film (Gold Special Jury Award) at the Houston International Film Festival and was nominated for Best Picture at the Hollywood Film Festival. It also screened at Raindance Film Festival.

He has taught his 22-Step Great Screenwriting and Genre classes to over 20,000 students worldwide. He has also worked as a story consultant and script doctor for Disney Studios, Sony Pictures, FOX, HBO, Alliance Atlantis, and Cannell Studios. In Europe, Mr. Truby has consulted for the BBC, RAI, LUX, TV4 and MTV Sweden. His students include the writer/director/or producer for the following films:

Shrek, Pirates of the Caribbean, The Mask of Zorro, Nightmare on Elm Street, Outbreak, Scream, Sleepless in Seattle, Back to School, The Addams Family, Kiss of the Spider Woman, Beetlejuice, Valley Girl, The Negotiator, Star Wars.